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Emissions of nitrous oxide (N2O), a potent greenhouse gas, from farmland have been recognized to be affected by soil pH and nitrogen (N) fertilizer application. However, the interactive effects of soil pH and N fertilizer type on N2O emissions and their influencing mechanism are poorly understood. A field experiment was conducted to elucidate the impacts of synthetic fertilizer and manure on soil properties and N2O fluxes along a soil acidity gradient (soil pH = 6.8, 6.1, 5.2, and 4.2) in the Huai River Basin, and a lab incubation experiment was performed to understand the underlying mechanisms of changed N2O flux. Low soil pH inhibited the ammonia-oxidizing bacteria abundance and thereby reduced the N2O production by nitrification under both synthetic fertilizer and manure application. The N2O production by denitrification was also reduced with declining soil pH, likely due to the decreased nirS and nirK abundances, and lower NO3−. However, low soil pH reduced the nosZ abundance and increased (nirS + nirK)/nosZ ratio, resulting in the increased N2O/(N2O + N2) ratio. Finally, with the decreased nitrification and denitrification, soil N2O emission was significantly reduced with declining soil pH regardless of fertilizer types. Compared with synthetic fertilizer, manure application increased soil nutrients (total N, dissolved organic C, and NO3−), nirK abundance, and (nirS + nirK)/nosZ ratio in the soils with pH of 5.2 and 4.2, thereby promoting N2O production by denitrification and N2O/(N2O + N2) product ratio in acidic soils. Consequently, soil N2O emission was increased with manure application in acidic soils. This study provides novel insight and improves our understanding of how soil pH regulates nitrification, denitrification, and N2O emissions from soils amended with chemical fertilizer and manure, which gives guidance on developing N management strategies for sustainable production and N2O mitigation in acid soils. [ABSTRACT FROM AUTHOR]

The regulatory gate hypothesis considers that soil organic C mineralization is a two-step process, where stable C is firstly transformed abiotically and is only then a microbially available substrate. The mechanisms involved in the abiotic conversion of non-microbially available to available soil organic C remain largely unknown. We conducted a perfusion experiment using a repeated fumigated-incubated soil and the corresponding fresh soil. We found that repeated fumigation-incubation significantly decreased soil microbial ATP to 0.22 nmol g−1 soil, 10% of that in the fresh soil, and significantly destroyed microbial composition and diversity. However, it had little influence on the soil CO2-C evolution rate after the flush of fumigant-killed dead biomass (8 μg CO2-C g−1 soil day−1) or dissolved organic C (DOC) concentration (about 7 μg C g−1 soil) and composition during long-term perfusion. We conclude that soil CO2 evolution rate and DOC generation were not regulated by the size or composition of the soil microbial communities. This is in support of the regulatory gate hypothesis. We suggest that abiotic processes in soil organic C mineralization need to be considered more and studied further. [ABSTRACT FROM AUTHOR]

Labile carbon (C) input to soils is expected to affect soil organic matter (SOM) decomposition and soil organic C (SOC) stocks in temperate coniferous forests. We hypothesized that the SOM decomposition rate, C content in soil fractions, and microbial and faunal abundance and activity were increased along the gradient in labile C input around wood ant nests. Three distances from the nest that differed in annual labile C input to soil were selected: 4 m with 6379 mg C m−2, 30 m with 9060 mg C m−2, and 70 m with 9215 mg C m−2. Soil from the organic horizon (Oe+Oa), surface mineral horizon (A), and subsoil mineral horizon (B) was analyzed for C content in soil fractions and for activity and abundance of soil microorganisms and fauna. In addition, a 1-year litter-bag and soil-bag decomposition experiment was conducted. Although the rate of soil decomposition did not differ along the labile C input gradient, the rate of litter decomposition in the B horizon increased as labile C input increased with distance from the nest. Correspondingly, the C content in bulk soil and in the labile and less-protected soil fractions in the B horizon decreased as labile C input increased. We infer that, because the O and A horizons are less C-limited than the B horizon, the changes in the labile C input along the gradient affected the B horizon more than the surface O and A horizons. However, soil microbial and faunal activity and abundances were not consistently affected by the gradient. Apparently, C stocks in soil fractions are more important for microbial and faunal communities than labile C inputs. Although the results indicate that SOC content changes very slowly in the coniferous forest soil of the current study, increases in the input of natural labile C leads to decreases in the SOC stock in the B horizon. By increasing the labile C input to temperate forest soils, future increases in atmospheric CO2 concentration may therefore lead to a significant loss of SOC in deep soil layers. [ABSTRACT FROM AUTHOR]

Straw mulching has been used to conserve soil water and sustain dryland crop yields, but the impact of the quantity and time of mulching on soil C fractions are not well documented. We studied the effects of various amounts and times of wheat (Triticum aestivum L.) straw mulching on soil C fractions at 0-10- and 10-20-cm depths from 2009 to 2017 in the Loess Plateau of China. Treatments were no mulching (CK), straw mulching at 9.0 (HSM) and 4.5 Mg ha−1 (LSM) in the winter wheat growing season, and straw mulching at 9.0 Mg ha−1 in the summer fallow period (FSM). Soil C fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). All C fractions at 0-10 and 10-20 cm were 8-27% greater with HSM and LSM than FSM and CK. Both SOC and POC at 0-10 cm increased at 0.32 and 0.27 Mg ha−1 year−1 with HSM and at 0.40 and 0.30 Mg C ha−1 year−1 with LSM, respectively, from 2009 to 2017. Winter wheat grain yield was lower with HSM and LSM, but total aboveground biomass was greater with HSM than other treatments. All C fractions at most depths were correlated with the estimated wheat root residue returned to the soil and PCM at 0-10 and 0-20 cm was correlated with wheat grain yield. Wheat straw mulching during the growing season increased soil C sequestration and microbial biomass and activity compared with mulching during the fallow period or no mulching, regardless of mulching rate, due to increased C input, although it reduced wheat grain yield. Continuous application of straw mulching over time can increase soil C sequestration by increasing nonlabile C fractions while decreasing labile fractions. Straw mulching at higher rate and mulching during the summer fallow period had no additional benefits in soil C sequestration. [ABSTRACT FROM AUTHOR]

Enzyme activity as a method for soil biochemistry and microbiology research has a long history of more than 100 years that is not widely acknowledged in terms of adherence to strict assay protocols and the interpretation of results. However, in the recent past, there is a growing lack of recognition of the historic advancements among researchers that use soil enzymology. Today, many papers are being published that use methods that either do not follow exact protocols as originally vetted in the research literature or individual labs use their own method that has not been optimized for pH, co-factors, substrate concentrations, or other conditions. This is of particular concern for fluorogenic substrates and microplate methods. Furthermore, there is a lack of understanding of the origin and location of a given enzyme being studied. Notably, regardless of the enzyme, it is too often assumed that enzyme activity equals microbial activity-which is not the case for most hydrolytic enzyme assays. Because as established by Douglas McLaren in the 1950s, a considerable amount of activity can come from catalytic enzymes stabilized in the soil matrix but that are no longer associated with viable cells (known as abiontic enzymes). In summary, today, many papers are using imperfect methods and/or misinterpret enzyme activity data that at a minimum confounds cross paper studies and meta-analysis. However, most importantly, lack of historical perspectives and ignoring strict protocols cause redundancy and fundamentally undermine the discipline and understanding of soil microbiology/biochemistry when enzymology methods are used. [ABSTRACT FROM AUTHOR]

The soil organic matter (OM) content of soils in a long-term fertiliser field trial (Winchmore, New Zealand) are similar ( P > 0.05) despite >60 years application of different phosphorus (P) rates. As the net primary productivity increased with P addition, greater losses of carbon (C) occur concomitantly with increased P fertility. Several hypotheses have been proposed to explain the mechanisms, including C leaching, increased earthworm activity or elevated rates of microbial activity. In this study, we found support for both direct and secondary effects of soil P on soil C through impacts on the soil microbial community. Microbial biomass, inferred through quantification of hot water extractable C, increased with soil P status and decreased with C/P ratio ( P < 0.001). However, the microbial biomass had no relationship with soil organic C content ( P = 0.485). Mineralisation of C substrates added to soil also increased with soil P status (total P, R = 0.84; P < 0.001). These results indicated potential conditioning of the microbial community for rapid C cycling. Utilisation of different C compounds was clustered by cophenetic similarity; a distinct group of ten carbon compounds was identified for which rates of mineralisation were strongly associated with soil P status and microbial biomass. However, this alteration of microbial community size and activity was not reflected in abundances of selected oligotrophic and copiotrophic taxa. As such, the alteration may be due to changes in the abundances of all taxa, i.e. a general community response. [ABSTRACT FROM AUTHOR]

As a consequence of climate change, a rising sea level results in increased saltwater intrusion and soil salinity problems in Bangladesh. The affected agricultural soils are characterized by temporarily high concentrations of soluble salts, low organic matter and reduced plant growth. Addition of organic matter can reduce the negative effects of salinity on soil microorganisms and positively influence nutrient cycling. Therefore, the benefit of rice straw addition to saline paddy soils on the microbial community and associated mineralization dynamics was investigated in a short-term aerobic laboratory incubation experiment. Soil samples were collected from two locations in the coastal area of Bangladesh previously prone to salinity. Both soils were incubated with rice straw (0, 25 and 50 % of the straw yield) and three levels of salinity (addition of 0, 15 and 50 mg NaCl g soil) for 28 days at 22 °C. Soil respiration was measured during incubation, and microbial properties, extractable C and inorganic N were determined at the end of the experiment. Increasing soil salinity only slightly decreased soil respiration and the ergosterol content, whereas the microbial biomass C was not consistently affected by increasing salinity and was generally at a very low level. Moreover, the bacterial-to-fungal abundance ratio increased with increasing salinity. Nitrification was only substantially reduced at extremely high salinity, indicating adaptation of the nitrifiers. On the other hand, addition of rice straw only slightly improved microbial properties and resulted in N immobilization. To improve soil fertility, addition of organic matter is crucial. Returning a portion of the rice straw to the soil only slightly contributes to this goal. Moreover, rice straw is a valuable resource used, e.g. as feed or burning material, making the return to the field a less economically viable option. Other sources of larger quantities and higher quality of organic matter should thus be preferred. [ABSTRACT FROM AUTHOR]

Understanding the decomposition processes of crop residues and the quantity of residue carbon (C) incorporated into soil organic C (SOC) pools in the soil is crucial for optimizing C management in agricultural systems. This study is highly valuable in Mollisols in northeastern China, where SOC is markedly decreasing. Soybean is a major crop in this region; however, the decomposition processes of soybean residues and their contributions to physically separated SOC pools remain unknown. Thus, a 150-day incubation experiment was conducted with different C-labelled residues of soybean, i.e., leaf, stalk, and root, incorporated into a Mollisol. The leaves had the highest decomposition rate. At the end of the incubation, cumulative respiration reached 7.76 mg CO-C g in the leaf-incorporated soil, but only 5.98 and 5.51 mg CO-C g was recorded for the stalk- and root-amended soils. Furthermore, similar trends were found for the microbial biomass C and dissolved organic C. Different residue sources greatly affected the residue-derived C incorporation in the SOC fractions, resulting in a ranking of root > stalk > leaf. The root-derived C incorporation values were 49.5, 17.2, and 5.0 g residue C kg in the coarse particulate organic C (POC), fine POC, and mineral-associated C (MOC) fractions, respectively, which were significantly higher than those for the stalk- and leaf-derived C. These results indicate that C input from roots can play an important role in C stability in this Mollisol by incorporating more C in the POC and MOC. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is currently considered a valuable technology for recycling of cattle slurry due to the production of biogas, a renewable source of energy. However, an appropriate management of the waste generated from anaerobic co-digestion of cattle slurry (digestate) is needed for the sustainability of the process. This paper shows the effects provoked on soil C and N mineralisation processes and on microbial biomass due to the addition of the digestate, the liquid and solid fractions obtained after separation of the digestate and the composted solid fraction of the digestate, in comparison with the effects of the non-digested cattle slurry. Composting was a very effective way of recycling the digestate, producing highly stable (mineralised-C reached only 3% of the added total organic-C) and hygienised material with an elevated fertilising potential (the net-N mineralisation accounted for 2% of total-N from compost). The agricultural use of cattle slurry, the digestate and the separated fractions should take into account their effects on soil C and N cycles: high decomposition rate (ranging from 25% to 50% of total organic-C) and partial N immobilisation in the soil. [ABSTRACT FROM AUTHOR]

The effects of addition of synthetic dairy factory effluent (DFE) containing phosphoric, nitric or acetic acids as cleaning agents on soil chemical and microbial properties and potential nutrient leaching from a dairy farm soil were studied in an 84-day open incubation/leaching experiment and in a short-term (24 days) closed incubation. Dairy factory effluent resulted in increased leaching of Na and Ca. The main anion leached in the DFE treatments was Cl, whilst in the control treatments it was NO. Leachates from DFE-treated soil had a greater electrical conductivity (EC) and lower pH than those from controls, but type of acid had little effect on the ionic content or composition of leachates. At the conclusion of the experiment, soils from DFE treatments showed a large accumulation of exchangeable Na, a reduction in exchangeable Ca and significant increases in microbial biomass C, basal respiration and β-glucosidase and arylsulphatase activities. Microbial activity was greatest from the acetic acid-DFE treatment. There was a significant increase in extractable P in the phosphoric acid-DFE treatment, but addition of nitric acid-DFE did not increase leaching of NO. In the short-term incubation experiment, EC increased over time in all treatments, while pH, microbial biomass and basal respiration decreased. During the first few days after a one-off DFE addition, there was an increase in both microbial biomass C and basal respiration, but these differences dissipated after 3 days. It was concluded that while additions of DFE have significant effects on chemical and microbial soil properties, a change in the form of acid present in the DFE has relatively minor effects on these properties. [ABSTRACT FROM AUTHOR]

The short-term effects of excessive NH-N on selected characteristics of soil unaffected (low annual N inputs) and affected (high annual N inputs) by cattle were investigated under laboratory conditions. The major hypothesis tested was that above a theoretical upper limit of NH concentration, an excess of NH-N does not further increase NO formation rate in the soil, but only supports accumulation of NO-N and gaseous losses of N as NO. Soils were amended with 10 to 500 μg NH-N g soil. In both soils, addition of NH-N increased production of NO-N until some limit. This limit was higher in cattle-affected soil than in unaffected soil. Production of NO increased in the whole range of amendments in both soils. At the highest level of NH-N addition, NO-N accumulated in cattle-affected soil while NO-N production decreased in cattle-unaffected soil. Despite being statistically significant, observed effects of high NH-N addition were relatively weak. Uptake of mineral N, stimulated by glucose amendment, decreased the mineral N content in both soils, but it also greatly increased production of NO. [ABSTRACT FROM AUTHOR]

Intensive agriculture (IA) is widespread in South Italy, although it requires frequent tillage, large amounts of fertilizers and irrigation water. We have assessed the efficacy of reversing IA to sustainable agriculture (SA) in recovering quality of a typical South Italy soil (Lithic Haploxeralf). This reversion, lasting from 2000 to 2007, replaced 75% of nutrients formerly supplied inorganically by farmyard manuring and reduced the tillage frequency. Several chemical and biochemical properties, functionally related to C and N mineralisation–immobilisation processes and to P and S nutrient cycles, were monitored annually from 2005 to 2007 in the spring. Reversing IA to SA decreased soil bulk density, almost doubled the soil organic matter (SOM) as favoured the immobilisation of C and N, increased most soil microbial indicators but decreased contents of nitrate, mineral N and K2SO4-extractable C. The K2SO4-extractable C/K2SO4-extractable organic N ratio suggested that substrate quality rather than the mass of readily available C and N affected biomass and activity of soil microflora. Also, the largely higher 10-day-evolved CO2–C-to-inorganic N ratio under SA than IA indicated that higher C mineralisation, associated with higher microbial biomass N immobilisation, occurred under SA than IA. Decreases in most soil enzyme activities under IA, compared to SA, were much higher than concomitant decreases in SOM content. Soil salinity and sodicity were always higher in IA than SA soil, although not critically high, likely due to the intensive inorganic fertilisation as irrigation waters were qualitatively and quantitatively the same between the two soils. Thus, we suggest that the cumulative small but long-term saline (osmotic) and sodic (dispersing) effects in IA soil decreased the microbial variables more than total organic C and increased soil bulk density. [ABSTRACT FROM AUTHOR]

Manufactured soil for landscaping purposes was produced by composting for 6 weeks (1) municipal green waste alone, (2) green waste amended with 25% v/v poultry manure, or (3) green waste immersed in, and then removed from, a mixture of liquid grease trap waste/septage. Composting temperatures increased most rapidly and reached highest values (78°C) in the grease trap/septage-amended green waste. In comparison with green waste alone, addition of poultry manure prolonged the period of elevated temperatures and increased the maximum temperature attained from 52°C to 61°C. Following composting, each of the materials was split into (1) 100% compost, (2) 80% compost plus 20% v/v soil, and (3) 70% compost plus 20% soil plus 10% coal fly ash. Addition of poultry manure or grease trap/septage to green waste prior to composting increased bulk density and reduced total porosity of the composted product. Addition of soil, or soil and ash, to composts increased bulk density, reduced total porosity, decreased percentage macropores, and increased percentage mesopores and available water-holding capacity. Bicarbonate-extractable P, exchangeable NH4+ and NO3-, electrical conductivity (EC), soluble C, soluble C as a percentage of organic C, basal respiration, and metabolic quotient were all markedly greater in the grease trap/ septage-amended than poultry manure-amended or green waste alone treatments. Values for extractable P and EC were considered large enough to be damaging to plant growth and germination index (GI) of watercress was less than 60% for all grease trap/septage composts. Extractable P and EC were also high, and GI was <100%, in the green waste alone and poultry manure-amended green waste alone treatments. Addition of soil or soil and ash to these composts resulted in GI values >100%. [ABSTRACT FROM AUTHOR]

The response of a series of soil microbial, chemical, and enzymatic chacacteristics to two different tillage systems (conventional and no-tillage) and two crop rotations (continuous corn and soybean–corn) was evaluated in a long-term field experiment in the Rolling Pampas Region, Marcos Juárez, Córdoba, Argentina. The parameters studied included: oxidizable, soluble and respiration C, total and hydrolyzable N, microbial activity, ammonifiers, nitrifiers, cellulolytic and total microflora, protease and urease activities. After 15 years, oxidizable C, total N and hydrolyzable N showed no differences regarding no-tillage systems. However, differences were found regarding control soil. While soluble C did not exhibit any significant differences, respiration C and microbial activity were indicators sensitive to different treatments. Enzymatic activities, protease in particular, did reflect changes due to management. Fluctuation in microbial population counts were more related to plant residues than to tillage systems. [ABSTRACT FROM AUTHOR]

The effect of long-term waste water irrigation (up to 80 years) on soil organic matter, soil microbial biomass and its activities was studied in two agricultural soils (Vertisols and Leptosols) irrigated for 25, 65 and 80 years respectively at Irrigation District 03 in the Valley of Mezquital near Mexico City. In the Vertisols, where larger amounts of water have been applied than in the Leptosols, total organic C (TOC) contents increased 2.5-fold after 80 years of irrigation. In the Leptosols, however, the degradability of the organic matter tended to increase with irrigation time. It appears that soil organic matter accumulation was not due to pollutants nor did microbial biomass:TOC ratios and qCO2 values indicate a pollutant effect. Increases in soil microbial biomass C and activities were presumably due to the larger application of organic matter. However, changes in soil microbial communities occurred, as denitrification capacities increased greatly and adenylate energy charge (AEC) ratios were reduced after long-term irrigation. These changes were supposed to be due to the addition of surfactants, especially alkylbenzene sulfonates (effect on denitrification capacity) and the addition of sodium and salts (effect on AEC) through waste water irrigation. Heavy metals contained in the sewage do not appear to be affecting soil processes yet, due to their low availability. Detrimental effects on soil microbial communities can be expected, however, from further increases in pollutant concentrations due to prolonged application of untreated waste water or an increase in mobility due to higher mineralization rates. [ABSTRACT FROM AUTHOR]

Maize ( Zea mays L.) and sunflower ( Helianthus annuus L.) grown on a calcareous soil showed poor growth and/or were chlorotic in spite of abundant Fe in the roots. It has been hypothesized that microbial siderophores chelate Fe (III) in the soil, and that in this form Fe is transported towards the root apoplast. On the calcareous soil, total and apoplastic root Fe concentrations were high, probably because of a high apoplastic pH depressing Fe (III)-reductase activity and thus the Fe2+ supply to the cytoplasm. On the acidic soil, total and apoplastic root Fe concentrations were low, probably because of a low apoplastic pH favouring Fe (III) reduction, hence plants showed no Fe-deficiency symptoms. The main objective of the present work was to investigate the role of microbial soil activity in plant Fe acquisition. For this purpose, plants were grown under sterile and non-sterile conditions on a loess loam soil. Plants cultivated under non-sterile conditions grew well, showed no Fe-deficiency symptoms and had fairly high Fe concentrations in the roots in contrast to plants grown in the sterile medium. Low root and leaf Fe concentrations in the axenic treatments indicated that the production of microbial siderophores was totally suppressed. Accordingly, sunflowers were severely chlorotic and this was associated with very poor growth, whereas in maize only growth was drastically reduced. In maize under sterile conditions, root apoplastic and total Fe concentrations were not as low as in sunflowers, which may have indicated that phytosiderophores produced in maize partly sustained Fe acquisition, but due to poor growth were not as efficient in supplying Fe as microbial activity under natural conditions. It may be therefore assumed that in natural habitats soil microbial activity is of pivotal importance for plant Fe acquisition. [ABSTRACT FROM AUTHOR]

The effects of 5 years of continuous grass/clover (Cont grass/clover) or grass (Cont grass) pasture or 5 years of annual grass under conventional (Ann grass CT) or zero tillage (Ann grass ZT) were compared with that of 5 years of continuous barley (LT arable) on a site which had previously been under arable crops for 11 years. For added comparison, a long-term grass/clover pasture site (LT past) nearby was also sampled. Soil organic C (Corg) content followed the order LT arable=Ann grass CTmic), basal respiration, flourescein diacetate (FDA) hydrolytic activity, arginine ammonification rate and the activities of dehydrogenase, protease, histidase, acid phosphatase and arylsulphatase enzymes were broadly similar to those for Corg. For Cmic, FDA hydrolysis, arginine ammonification and the activities of histidase, acid phosphatase and arylsulphatase, the percentage increase caused by 5 years of continuous pasture (in comparison with LT arable) was 100–180%, which was considerably greater than that for organic C (i.e. 60%). The microbial metabolic quotient (qCO2) was higher for the two treatments which were mouldboard ploughed annually (LT arable and Ann grass CT) than for the undisturbed sites. At the undisturbed sites, Corg declined markedly with depth (0–15 cm) and there was a similar stratification in the size and activity of Cmic and enzyme activity. The microbial quotient (Cmic/Corg) declined with depth whilst qCO2 tended to increase, reflecting a decrease in the proportion of readily available substrate with depth. [ABSTRACT FROM AUTHOR]

Non-tilled and tilled plots on a spodosol (Corg 0.65–1.70%; pH 4.1–4.5) and a mollisol (Corg 3.02–3.13%, pH 4.9–5.3), located in the European region of Russia, were investigated to determine variances in soil microbial biomass and microbial community composition. Continuous, long-term management practices, including tillage and treatment with inorganic fertilizers or manure, were used on the spodosol (39 years) and mollisol (22 years). Total microbial biomass (Cmic), estimated by the substrate-induced respiration (SIR) method, and total fungal hyphae length (membrane filter technique) were determined seasonally over a 3-year period. Long-term soil management practices (primarily tillage and fertilizer application) led to decreases in total microbial biomass (80–85% lower in spodosol and 20–55% lower in mollisol), decreases in the contribution of Cmic to Corg (2.3- to 3.5-fold lower in spodosol and 1.2- to 2.3-fold lower in mollisol), and 50–87% decreases in total fungal hyphae length compared to non-tilled control plots. The contribution of fungi to total SIR in virgin mollisol and fallow spodosol plots was approximately 30%. However, the contribution of fungi to SIR was approximately two times greater in tilled spodosol plots compared to a fallow plot. In contrast, the contribution of fungi to SIR in tilled plots of mollisol was less (1.4–4.7 times) than for a virgin plot. In summary, long-term soil management practices such as tillage and treatment with organic or inorganic fertilizers are important determinants of soil microbial biomass and the contribution of fungi to total SIR. [ABSTRACT FROM AUTHOR]

The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (Corg) content under long-term pasture (>60 years) was in the range of 55 g C kg–1 to 65 g C kg–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (Cmic) and basal respiratory rate. The microbial quotient (Cmic/Corg) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg–1 to 15 g C kg–1 but the microbial metabolic quotient (basal respiration/Cmic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg–1. Soils with an organic C content above 45 g C kg–1 had been under pasture for at least 30 years. At the same Corg content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. [ABSTRACT FROM AUTHOR]

Earthworm activity may be an important cause of spatial and temporal heterogeneity of soil properties in agroecosystems. Structures known as “earthworm middens,” formed at the soil surface by the feeding and casting activities of some earthworms, may contribute significantly to this heterogeneity. We compared the temporal dynamics of carbon (C), nitrogen (N), and microbial acitivity in Lumbricus terrestris middens and in surrounding non-midden (bulk) soil during the spring, when seasonal earthworm activity was high. We sampled soil from middens and bulk soil in a no-till cornfield on four dates during May and June 1995. Soil water content and the weight of coarse organic litter (>2mm) were consistently higher in middens than in bulk soil. Total C and N concentrations, C:N ratios, and microbial activity also were greatest in midden soil. Concentrations of ammonium-nitrogen and dissolved organic N were greater in middens than in bulk soil on most dates, suggesting accelerated decomposition and mineralization in middens. However, concentrations of nitrate were usually lower in middens, indicating reduced nitrification or increased leaching and denitrification losses from middens, relative to bulk soil. Fungal activity, as well as total microbial activity, was consistently greater in middens. The contribution of fungae to overall microbial activity differed significantly between middens and bulk soil only on one date when both soils were very dry; the contribution of fungae to microbial activity was lower in the middens on this date. We conclude that the midden-forming activity of L. terrestris can be a major determinant of spatial heterogeneity in some agricultural soils, and that this can potentially affect overall rates of soil processes such as organic matter decomposition, N mineralization, denitrification, and leaching. [ABSTRACT FROM AUTHOR]

Saline environments have a natural bacterial flora, which may play a significant role in the economy of these habitats. The natural saline environments (usually containing salinity equivalent to 4–30% NaCl) are aquatic (e.g. salt marshes) or terrestrial (e.g. saline lands). Saline environments include an increasing area of salt-affected cultivated soils throughout the world. These environments contain various ions which may interfere with uptake of water and which may be toxic to a large number of organisms. Saline environments harbour taxonomically diverse bacterial groups, which exhibit modified physiological and structural characteristics under the prevailing saline conditions. The majority of these bacteria can osmoregulate by synthesizing specific compatible organic osmolytes such as glutamine, proline and glycine betaine and a few of them accumulate inorganic solutes such as Na+, K+ and Mg2+. The morphology of the bacteria is usually modified, cells are usually elongated, swollen and showing shrinkage, in addition to changes in the cell and cytoplasmic volume. The chemical composition of membranes may also occasionally be modified, and the synthesis pattern of proteins, lipids, fatty acids and polysaccharides may change with a moderate increase in salinity. However, ultrastructural alterations in cells of halophilic bacteria have not been reported, and profound changes in cellular properties of these bacteria only occur at concentrations above 2 MNaCl. Evidence has accumulated that the bacteria are essential elements in the saline environment because of their activity such as degradation of plant remains, nitrogen fixation and production of active metabolites. [ABSTRACT FROM AUTHOR]

The variation in stubble decomposition due to fertilizer incorporation was determined in a typical Argiudoll of the Argentinian rolling pampas. The experiment was conducted for 15 years, which included a no till system under maize ( Zea mays L.) and a soybean rotation ( Glycine max) with 0 and 45kg N ha–1 nitrogen fertilization treatments, called NFS and FS, respectively. A higher proportion of residues with a high N content was found in the FS plots. The different substrate quality proved to be the regulating factor for mineralization. This activity was indicated by the increase in soil microbial activity and soil carbon light fraction in FS compared to NFS. This carbon light fraction mineralizes rapidly but does not contribute to the most stable components, which are related to synthesis and polymerization of humic acids. No significant differences in humic acid content were found. [ABSTRACT FROM AUTHOR]

In sandy gleyic soils with a low groundwater table under arboriculture in Northwest Germany, a wide variation of groundwater pollution by pesticides has been observed. We therefore examined data on microbial activity and soil organic matter composition by wet chemistry, cross-polarization magic-angle spinning and C nuclear magnetic resonance, and pyrolysis-field ionization mass spectromy. However, neither microbial activity nor the soil organic matter composition of cultivated topsoils explained the differences in xenobiotic leaching into the groundwater. Data from Anthrosols suggested that these soils have a higher capacity for pesticide bonding because of high amounts of aromatic and carboxylic C moieties in the soil organic matter. However, despite the same pesticide inputs and time of application, the leached output from these soils was higher than that from the Podzols. Initial data from subsoil investigations suggest that the presence of a spodic horizon most likely reduces groundwater pollution by pesticides. Studies to assess fixation capacity and desorption kinetics in Bh horison seem warranted. [ABSTRACT FROM AUTHOR]

Modifications of soil microbiological activity by the addition of municipal solid-waste compost were studied in laboratory incubations. Three composts were compared, one lumbricompost and two classical composts with different maturation times. Organic C mineralization and nine enzyme activities (dehydrogenase, peroxidase, cellulase, β-glucosidase, β-galactosidase, N-acetyl-β-glucosaminidase, protease, amidase, and urease) were determined in the composts and the amended soil. Initial enzyme activities varied in the soil according to the sampling date (winter or summer) and were greater in the composts than in the soil, except for urease. Generally, the youngest compost exhibited greater activity than the oldest one. In the amended soil, the composts did not increase enzyme activity in an additive way. Dehydrogenase, the only strictly endocellular enzyme, was the only one for which the activity in the amended soil increased significantly in proportion to the addition of compost. During the incubations, C mineralization and dehydrogenase activity were significantly correlated, indicating that dehydrogenase was a reliable indicator of global microbial activity. Peroxidase activity in the soil remained constant, but increased in the composts and amended soil. Addition of the oldest compost had no effect on the activity of the C cycle enzymes, but the youngest compost increased creased soil activity at the higher application rate. Enzymes of the N cycle were stimulated by all compost amendments, but the increase was only transient for amidase and urease. Lumbricomposting had no marked effect on compost enzyme activity, either before or during the incubation. [ABSTRACT FROM AUTHOR]

Microbial parameters appear very useful in monitoring soil pollution by heavy metals, but no single microbial parameter can be used universally. Microbial activities such as respiration, C and N mineralization, biological N fixation, and some soil enzymes can be measured, as can the total soil microbial biomass. Combining microbial activity and population measurements (e.g., biomass specific respiration) appears to provide more sensitive indications of soil pollution by heavy metals than either activity or population measurements alone. Parameters that have some form of 'internal control', e.g., biomass as a percentage of soil organic matter, are also advantageous. By using such approaches it might be possible to determine whether the natural ecosystem is being altered by pollutants without recource to expensive and long-running field experiments. However, more data are needed before this will be possible. Finally, new applications of molecular biology to soil pollution studies (e.g., genetic fingerprinting) which may also have value in the future are considered. [ABSTRACT FROM AUTHOR]

The Dehérain long-term field experiment was initiated in 1875 to study the impact of fertilization on a wheat-sugarbeet rotation. In 1987, the rotation was stopped to be replaced by continuous maize. Crop residues were soil-incorporated and the mineral fertilization was doubled in some plots. The impact of those changes on the microbial biomass and activity are presented. In spring 1987, the soil was still in a steady-state condition corresponding to the rotation. The microbial biomass was correlated with total organic C and decreased in the order farmyard manure>mineral NPK>unfertilized control. Microbial specific respiratory activity was higher in the unfertilized treatments. The soil biomass was closely related to soil N plant uptake. In 1989, after 2 years of maize and crop residue incorporation, the steady-state condition corresponding to the previous agricultural practices disappeared. So did the relationship between the biomass and total organic C, and the soil N plant uptake. Biomass specific respiratory activity increased because of low efficiency in the use of maize residues by microbes under N stress. [ABSTRACT FROM AUTHOR]

Microbial biomass C and activity were determined in six forest soils along a gradient in physical and chemical climate in Europe. Both parameters were measured microcalorimetrically. The upper 22 cm of the soils were sampled in undisturbed columns (24 cm deep). Measurements were made in homogenized samples of the different surface organic horizons (Ol, Of, Oh) and the mineral horizons (Ah, Aeh, Bv) down to 22 cm. On a mass basis values for both the biomass and the activity showed an exponential decrease with depth in all soils. Expressed on a volume basis these relationships varied with soil pH. in the strongly acidified soils most of the microbial biomass and activity was located in the forest floor. In less acidified soils both parameters were highest in the mineral soil. Further relationships between biomass and activity and between soil chemical properties showed significant positive correlations with exchangeable Ca, Mg, Ca/Al and negative correlations with Al. There were no significant correlations with exchangeable cations in less acidified soils. It was calculated that the microbial biomass is more affected by soil chemistry than activity. The caloric quotient ( qW) is a good parameter for determining the ecophysiological state of microorganisms in acidified soils. [ABSTRACT FROM AUTHOR]

The decomposition of oak leaf litter was studied by means of a litterbag experiment in an oak forest in the Netherlands. The contribution of microbial activity and leaching to weight loss and element dynamics during the first 6 weeks of decomposition was investigated by means of frequent respiration measurements and extractions of the litter and by a qualitative comparison of throughfall and litter percolation water chemistry. The oak-leaf litter lost 9.3% of its initial dry weight during the first 6 weeks. In total, 90% of this observed weight loss was explained by the processes studied. About 5.9% (64% of the total) of this weight loss was attributed to microbial tespiration and 0.5% (5%) to the loss of inorganic solutes. Leaching of dissolved organic compounds was estimated to account for 2.0% (21%). The results indicated a fast leaching of K and Cl out of the fresh litter during the first 2 weeks, while Mg, Fe, Mn, Si, ortho P, and dissolved organic N were released at a much lower rate. At the same time, small amounts of H, NHand NOwere retained in the litter. [ABSTRACT FROM AUTHOR]

A sensitivity-resistance index was developed, and proved to be a very sensitive biomonitor of soil pollution with heavy metals. The index was developed by a step-by-step approach. Ultimately, the bacterial soil microflora was divided into three groups, senstivive, tolerant, and resistant microflora. Zn and Cd sensitivity was defined as no growth occurring in the presence of 5 and 0.5 mg l of these metals, respectively, while resistance was defined as distinct growth in the presence of 50 and 16 mg l, respectively. The sensitivity: resistance ratio of a referent clay soil (0.57 mg Cd kg and 140 mg Zn kg) was 0.53, but for polluted (6 mg Cd kg + 670 mg Zn kg) clay soil, the ratio was 0.24. For a referent (0.06 mg Cd kg + 12 mg Zn kg) sandy soil the sensitivity: resistance ratio was 1.50 whereas polluted (2.3 mg Cd kg + 252 mg Zn kg) sandy soil had a ratio 0.19. The ecological value of the sensitivity-resistance lies in its capacity to reflect potential deradation of aromatic compounds. It has been shown repeatedly that sensitive bacteria grow significantly better on a range of selected aromatic compounds. It has been speculated that resistance fo heavy metals may reduce the bioremediation capacity of soil towards chlorinated aromatics and polyaromatic hydrocarbons. [ABSTRACT FROM AUTHOR]

Enzyme assays, once used primarily to collect descriptive information about soils, have become useful techniques for monitoring microbial activity and uncovering the mechanisms that underlie microbial processes. The simplest paradigm is that decomposition and nutrient cycling are emergent consequences of extracellular enzyme activities that are regulated directly by site-specific factors such as temperature, moisture and nutrient availability, and secondarily by litter chemistry through adsorption, inhibition and stabilization processes. In application, enzyme techniques are employed on three scales of resolution. On the largest scale, assays for ubiquitous enzymes such as phosphatase, esterase, and dehydrogenase are used as general measures of microbial activity. At higher resolution, enzyme specificity is exploited to monitor activity related to specific aspects of macronutrient cycling. At the highest resolution, the enzymatic mechanisms by which microorganisms interact with their environment are addressed. [ABSTRACT FROM AUTHOR]

Laboratory microcosms were used to study microbial populations and biomasses developing in fragmented litter of Pinus nigra Arnold var. nigra (A. et G.). Direct observations (fungal standing crop and fluorescein-stainable mycelia), litter enzyme analyses (cellulase and dehydrogenase), and measurements by physiological methods (microbial CO production and total microbial, fungal, and bacterial viable biomasses) were made at 3-week intervals for 15 weeks. Most variables showed great changes during this period, which were ascribed to a rise in litter moisture content during the initial phase of the experiment, and to substrate depletion towards its final phase. The addition of the collembolan Tomocerus minor (Lubbock) for 1 week enhanced cellulase activities by 4%. When the animals were introduced after 6 weeks, the fungal standing crop was enhanced, and the percentage of fluorescein-stainable mycelia was reduced. Dehydrogenase activity was increased by grazing when the microbial population had been established for 9 weeks or longer. Eucaryotic and procaryotic substrate-induced respiration were positively correlated, which was explained by partial segregation of resources for the two groups. Litter cellulase and dehydrogenase activity showed correlations by other techniques, indicating their suitability as parameters for microbial activity in general, and for the collembolan grazing impact on microbial activity in particular. [ABSTRACT FROM AUTHOR]

We have developed a simple method for the determination of gaseous compounds that reflect microbial activity in soil, as affected by factors such as the presence of an organic amendment (peat) or a variation in soil moisture. The method is based on a gas chromatographic analysis of the headspace of vials containing the soil under examination. A single gas chromatograph can detect up to 10 different gases. As expected, after peat was added to the soil, CO evolution and O uptake increased significantly. Positive relationships were found between the evolution of NO, and soil moisture and the amount of peat added to the soil. Both the these variables influenced the CO:O ratio. The results given by this method show high reproducibility. [ABSTRACT FROM AUTHOR]

Specialized ectomycorrhizal fungi form dense mats in forest soils that have different enzyme levels, higher respiration rates, more biomass, different soil fauna, and different soil chemistry compared with adjacent soils not obviously colonized by these mats. In this study, mats formed by two genera of fungi collected in three locations were compared with a wide range of measurements. Per cent moisture, pH, chloroform fumigation-flush C, anaerobic N mineralization, exchangeable ammonium, and respiration, N fixation, and denitrification rates were compared between soils or litter colonized by ectomycorrhizal mat-forming fungi and adjacent non-mat material. Significant differences were observed between the two genera of mat-forming fungi and also between mats formed primarily in mineral soil and those formed in litter. These differences suggest that different mat-forming fungi perform different functions in forest soils and that these fungi function differently in mineral soil compared with litter. [ABSTRACT FROM AUTHOR]

Conditions for a rapid, precise [100 μg iodonitrotetrazolium chloride (INT)-formazan ml assay mixture], and easily reproducible assay of potential soil dehydrogenase activity are described, using 2( p-iodophenyl)-3-( p-nitrophenyl)-5-phenyl tetrazolium chloride (iodonitrotetrazolium chloride, INT) as the substrate. Reduced iodonitrotetrazolium formazan (INTF) was measured by spectrophotometry (464 nm) after extraction with N,N-dimethylformamide and ethanol. With this method, the coloured complex formed is highly stable. The effects of pH, buffer concentration, temperature, substrate concentration, amount of soil weight, and reaction time on dehydrogenase activity were investigated. The rate of substrate hydrolysis was proportional to soil weight; the optimal INT reduction was achieved with 1 M TRIS buffer (pH 7.0) at 40 °C. It was possible to determine the biotic and abiotic substrate reduction by comparing assays of autoclaved and unsterile soil samples. Different investigations have confirmed that the intracellular enzyme is highly correlated with the microbial biomass, and indicate that this activity is suitable as an indirect parameter of microbial biomass, measurement. [ABSTRACT FROM AUTHOR]

The functional roles of the fungivorous collembolan Tomocerus minor and the detritivorous isopod Philoscia muscorum during the decomposition of Pinus nigra needles were studied in mesocosms filled with two different types of F1 litter, obtained from two different forest soils. The effects of the animals on the availability of K, Ca, NO, NH, and POand on the respiration, dehydrogenase, and cellulase activity of microorganisms were measured over one growing season. The animals were introduced into the F1 litter in three densities. The most important animal effect was a buffering effect, in that addition of the animals increased nutrient availability and microbial activity where the corresponding values in control mesocosms without animals were low, and decreased the nutrient availability and microbial activity where control values were high. This effect occurred for both species and was most evident in the substrate with the highest temporal fluctuations. The effects on nutrient availability are attributed to an animal effect on the activity of and successional stage reached the microbial community, with NHavailability seen as the most important factor. The concept of functional groups in relation to these animal effects is discussed. [ABSTRACT FROM AUTHOR]

The impact of protozoan grazing on the dynamics and mineralization of C- and N-labelled soil organic material was investigated in a microcosm experiment. Sterilized soil was planted with wheat and either inoculated with bacteria alone or with bacteria and protozoa or with bacteria and a 1:10 diluted protozoan inoculum. C−CO formation was continuously monitored. It served as an indicator of microbial activity and the respiration of soil organic C. The activity of protozoa increased the turnover of C-labelled substrates compared to soil without protozoa. The accumulated C−CO evolved from the soils with protozoa was 36% and 53% higher for a 1:10 and for a 1:1 protozoan inoculum, respectively. Protozoa reduced the number of bacteria by a factor of 2. In the presence of protozoa, N uptake by plants increased by 9% and 17% for a 1:10 and a 1:1 protozoan inoculum, respectively. Both plant dry matter production and shoot: root ratios were higher in the presence of protozoa. The constant ratio of N: N in the plants for all treatments indicated that in the presence of protozoa more soil organic matter was mineralized. Bacteria and protozoa responded very rapidly to the addition of water to the microcosms. The rewetting response in terms of the C−CO respiration rate was significantly higher for 1 day in the absence and for 2 days in the presence of protozoa after the microcosms had been watered. It was concluded that protozoa improved the mineralization of N from soil organic matter by stimulating the turnover of bacterial biomass. Pulsed events like the addition of water seem to have a significant impact on the dynamics of food-chain reactions in soil in terms of C and N mineralization. [ABSTRACT FROM AUTHOR]

Using microcosms containing decomposing Pinus nigra litter, the effects of introducing two species of soil arthropods, the fungivorous collembolan Tomocerus minor and the detritivorous isopod Philoscia muscorum, have been studied. The effects of these animals on microbial respiration, on dehydrogenase and cellulase activity, and on the concentration of exchangeable macronutrients (Ca, Mg, K, NO, NH, PO) were measured. Both species enhanced microbial activity and the concentration of exchangeable nitrate, ammonium, and phosphate. Ca and Mg concentrations were lowered in the microcosms with animals. The differences between the two species were mainly quantitative, and it appears that the effect of isopods is direct, whereas the collembolans show direct and indirect effects. Positive effects of the presence of animals were found when microbial activities or concentrations of exchangeable nutrients in microcosms without animals were low; negative effects were found when they were relatively high. Thus, soil arthropods have a buffering role in soil processes. These results ae discussed against a background of a supposed succession of sugar fungi/bacteria to more slowly growing decomposing fungi. [ABSTRACT FROM AUTHOR]

Two borings (20 m depth) were performed in a sandy-clayey soil over a limestone bed and in a sandy soil with lumps of clay in some depths. Bacteria were found in the deeper soil layers of both profiles. The methods used to detect bacteria were those normally used for topsoil layers, plate counts of bacteria, ATP content, and direct microscopy. Measurements of CO evolution showed that the potential for bacterial activity was present in all depths of the two profiles. However, the activity was strongly dependent on the presence of easily available organic C. An indication of the denitrification potential was obtained by measuring the NO evolution. Under aerobic incubation without the addition of glucose, NO was detected only in the topsoil. When glucose was added to the soil samples, NO was found at a low level in the deeper soil layers. Under anaerobic incubation, NO was detected in all deeper layers, and increased markedly when glucose was added to the soil samples. [ABSTRACT FROM AUTHOR]

Microbial biomass, activities of dehydrogenase, phosphatase, and urease, and numbers of ammonium oxidizers were determined at monthly intervals on soil samples obtained from an on-going tillage residue-management study during the summers of 1985 and 1986. The site was cleared of black spruce ( Picea mariana, Mill.) in 1979 and has been planted to spring barley ( Hordeum vulgare) since 1982. Tillage treatments were no-tillage or disked twice, and residuemanagement treatments were removal of stubble and loose straw or leaving all straw on the plots. Microbial biomass and enzyme activities were moderate to high in the Ap horizon but very low in the B horizon. There was no difference in any parameter measured due to tillage or residue management. In 1986, comparisons were made between the Ap horizon and the agricultural soil and the A horizon of the soil beneath an adjacent black-spruce forest. Total microbial biomass and enzyme activities were generally greater in the forest soil than in the agricultural soil. However, specific activity of the biomass was generally greater in the agricultural soil. Soil microbial biomass and urease activities of both agricultural and forest soils were similar to those reported for warmer climates, but dehydrogenase activity was higher and phosphatase was lower. [ABSTRACT FROM AUTHOR]

Two different soils were amended with C-labelled plant material and incubated under controlled laboratory conditions for 2 years. Half the samples were cropped with wheat ( Triticum aestivum) 10 times in succession. At flowering, the wheat was harvested and the old roots removed from the soil, so that the soil was continuously occupied by predominantly active root systems. The remaining samples were maintained without plants under the same conditions. During the initial stages of high microbial activity, due to decomposition of the labile compounds, the size of the total microbial biomass was comparable for both treatments, and the metabolic quotient (qCO-C = mg CO-C·mg Biomass C·h) was increased by the plants. During the subsequent low-activity decomposition stages, after the labile compounds had been progressively mineralized, the biomass was multiplied by a factor of 2-4 in the presence of plants compared to the bare soils. Nevertheless, qCO-C tended to reach similar low values with both treatments. The C-labelled biomass was reduced by the presence of roots and qCO-C was increased. The significance of these results obtained from a model experiment is discussed in terms of (1) the variation in the substrate originating from the roots and controlled by the plant physiology, (2) nutrient availability for plants and microorganisms, (3) soil biotic capacities and (4) increased microbial turnover rates induced by the roots. [ABSTRACT FROM AUTHOR]

Earthworms find in soil the energy, nutrient resources, water and buffered climatic conditions that they need. According to the food resource they exploit and the general environmental conditions, earthworms can be grouped into different functional categories which differ essentially in morphology, size, pigmentation, distribution in the soil profile, ability to dig galleries and produce surface casts, demographic profiles and relationships with the soil microflora. Soil characteristics are both the determinant and the consequence of earthworm activities, since these animals greatly influence the functioning of the soil system. When present, they build and maintain the soil structure and take an active part in energy and nutrient cycling through the selective activation of both mineralization and humification processes. By their physical activities and resultant chemical effects, earthworms promote short and rapid cycles of nutrients and assimilable carbohydrates. Thus earthworms represent a key component in the biological strategies of nutrient cycling in soils and the structure of their communities gives a clear indication of the type of soil system that they inhabit. [ABSTRACT FROM AUTHOR]

C and N mineralization potentials were determined, in a 12-week laboratory incubation study, on soil samples obtained from recently cleared land which had been cropped to barley for 4 years (field soils) and from nearby undisturbed taiga (forest soils). Treatments for the cropped soils were conventional and no-tillage with and without crop residues removed. An average of about 3% of the total C was evolved as CO from the field soils compared with > 10% and 4% for the upper (Oie) and lower (Oa) forest-floor horizons, respectively. Significantly more C was mineralized from the Ap of the no-till treatment with residue left on the surface than from the other field Ap horizons. Both forest-floor horizons showed rather long lag periods for net mineralization compared with the field soils, but at the end of the incubation, more mineral N was recovered from the forest Oie despite a rather wide C:N ratio, than from the field soils. After 12 weeks about 115, 200 and 20 μg mineral N/g soil were recovered from the field Ap, the forest Oie and the forest Oa horizons, respectively. Very little C or N was mineralized from the B horizon of the forest or the field soils. Nitrification was rapid and virtually complete for the field soils but was negligible for both forest-floor O horizons. [ABSTRACT FROM AUTHOR]

The relationship between the microbial activity in the soil and the effect of seed inoculation with the rhizopseudomonad strain 7NSK2 was evaluated in a series of pot experiments under greenhouse conditions. The microbial activity in plain soil, as measured by the respiratory activity, was significantly increased by the growth of the plants. Both the respiration rate of the microorganisms and the density of the bacteria and fungi in the bulk soil increased with increasing duration of the plant growth. Upon repeated short-term growth of plants on the same soil, a similar stimulation was noticed. The effect of seed inoculation on the growth of the maize cultivar Beaupré and the barley cultivar Iban was most pronounced in the microbiologically more active soils. The results suggest that the increase of the plant growth by seed inoculation is probably due to the inhibition of deleterious root microorganisms. [ABSTRACT FROM AUTHOR]